A. Field of the Invention
This invention relates generally to electrothermal printing, and more particularly to an improved printing ink to mark substrates such as paper, as well as apparatus and methods for use therewith.
B. Description of the Related Art
Traditional methods of printing utilize various types of long-run print forms such as gravure cylinders, offset plates or flexographic belts which carry a representation of the desired image (the so-called "signature") suitably recorded thereon. The print form is matched with a particular type of ink having physical characteristics compatible with those of the print form, thereby facilitating cycles of proper ink uptake and ink release. For example, in gravure printing, the gravure cylinder contains an imagewise pattern of depressions, so-called "cells", that accept ink from an ink-feed roller and subsequently deposit part of the ink under immense mechanical pressure onto a printing substrate (such as paper). This process generally requires low-viscosity inks having high solvent contents in order to promote the ready release of the ink from the gravure cells, even though large residual volumes of the ink will remain in the cells after these are pressed onto the print paper.
Lithographic offset printing, another much-used process, generally involves aluminum plates having imagewise signatures recorded thereon, the signatures residing in rasterized ink-accepting and ink-repellent areas comprising millions of print dots. In contrast to gravure systems, whether a given plate area will accept or repel ink is determined by its planar oleophobic and/or oleophilic surface characteristics rather than by the presence of cells. A lithographic offset plate is usually "imaged" by means of ultraviolet contact photography with a sheet of silver film; this process permits subsequent etching of the exposed plate raster-dot areas from an initial ink-accepting state into a water-accepting state, while leaving the unexposed dot areas in their original ink-accepting state. Lithographic inks are hydrophobic, and exhibit high, paste-like viscosities and contain relatively small amounts of solvent.
The primary disadvantages of these traditional printing methods stem from the tediousness and time-consuming lengthiness of recording the precise signature needed on the print member, and the inability to alter the imaged print member. Indeed, the print member is always discarded after its use. Due to the unalterable nature of the signature, the print form can only function repetitively; that is, it must print thousands of the same image signature in succession for later collation, cutting, and binding. Thus, the instantaneous variability of printed signatures, such as ascending page numbers, is impossible with traditional print forms, making short runs such as 100-1,000 prints quite uneconomical; this is particularly so when the long make-ready time for the press is considered. Furthermore, because traditional print methods require significant contact pressures between print form and printing substrate, the presses must be sturdy, precise and, consequently, very heavy and costly.
In order to avoid at least some of these difficulties, practitioners have developed alternative printing processes generally called "marking" methods. Ink-jet equipment, for example, completely eliminates the need for any print form, producing color images by ballistically jetting a serial sequence of ink droplets from a distance onto specially prepared paper (see, e.g., U.S. Pat. No. 4,593,292). The ink, which is generally volatile and low in viscosity, is usually furnished in liquid form, but sometimes it is provided in a solid form. The ink block is heated to fill a chamber from which a liquid droplet is subsequently ejected through a nozzle by means of a piezoelectric actuator; the droplet travels onto the paper where it impacts, loses its remaining heat and affixes. This process is relatively slow in terms of print speed, requires specially constructed paper substrates, is severly limited in terms of resolution, and lacks dot-by-dot gray-scale tonal capability. Furthermore, the resulting image typically remains entirely on top of the paper substrate in relief, and is thus susceptible to scratching and other physical damage unless solvent-based inks are used; these penetrate the paper to such a depth that the ink appears at the back of the paper.
Another alternative to traditional print-form-based printing is the thermal electrostatic method (see, e.g., U.S. Pat. No. 3,079,272 and British Patent No. 953,077). In this process, a discharge source deposits imagewise electrostatic charges onto a photoconductive plate or drum. The plate or drum is then brought into contact with a solid wax block, which consists of a carrier material in which electrostatic toner particles are dispersed. Heat or a solvent is applied to the block at the line of contact with the printing substrate, bringing the carrier into a molten state within which the charged particles can move freely. As the block is drawn along the substrate, it wets the entire surface with the wax; however, when the block reaches latent electrostatically charged image areas, the toner particles are slowly drawn onto the substrate by electrophoresis.
Once again, the need for a permanent print form is eliminated, in this case because of the ease with which a latent image pattern can be deposited onto an image cylinder or on the substrate itself. If the electrical resistivity of the toner is sufficiently high and the substrate carrier possesses good charge-retention characteristics, a single charge deposition can be re-used to produce a number of printed images; toners having low resistivity tend to dissipate the charged image areas by conduction.
The solid toners used in the above process consist of a carrier, such as wax, in which toner particles are dispersed. Thus, the carrier merely serves as a reservoir from which toner particles are continuously extracted during use, causing ultimate depletion of the reservoir. Because of the slowness of the electrophoretic process, the steady and non-uniform depletion of the reservoir, and the overall wax deposit on the entire image sheet, this process is considered impractical and therefore not used in commercial products.